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+From pgsql-sql-owner+M5999=candle.pha.pa.us=pgman@postgresql.org Mon Dec 17 01:39:56 2001
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+ Mon, 17 Dec 2001 01:21:17 -0500 (EST)
+To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
+ "MindTerm" <mindterm@yahoo.com>, pgsql-sql@postgresql.org
+Subject: Re: [SQL] performance tuning in large function / transaction
+In-Reply-To: <GNELIHDDFBOCMGBFGEFOIENDCAAA.chriskl@familyhealth.com.au>
+References: <GNELIHDDFBOCMGBFGEFOIENDCAAA.chriskl@familyhealth.com.au>
+Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+ message dated "Mon, 17 Dec 2001 12:06:14 +0800"
+Date: Mon, 17 Dec 2001 01:21:16 -0500
+Message-ID: <29547.1008570076@sss.pgh.pa.us>
+From: Tom Lane <tgl@sss.pgh.pa.us>
+Precedence: bulk
+Sender: pgsql-sql-owner@postgresql.org
+Status: OR
+
+"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
+> Is it true that the IN command is implemented sort of as a linked list
+> linear time search? Is there any plan for a super-fast implementation of
+> 'IN'?
+
+This deserves a somewhat long-winded answer.
+
+Postgres presently supports two kinds of IN (I'm not sure whether SQL92
+allows any additional kinds):
+
+1. Scalar-list IN: foo IN ('bar', 'baz', 'quux', ...)
+
+2. Sub-select IN: foo IN (SELECT bar FROM ...)
+
+In the scalar-list form, a variable is compared to an explicit list of
+constants or expressions. This form is exactly equivalent to
+ foo = 'bar' OR foo = 'baz' OR foo = 'quux' OR ...
+and is converted into that form by the parser. The planner is capable
+of converting a WHERE clause of this kind into multiple passes of
+indexscan, when foo is an indexed column and all the IN-list elements
+are constants. Whether it actually will make that conversion depends
+on the usual vagaries of pg_statistic entries, etc. But if it's a
+unique or fairly-selective index, and there aren't a huge number of
+entries in the IN list, a multiple indexscan should be a good plan.
+
+In the sub-select form, we pretty much suck: for each tuple in the outer
+query, we run the inner query until we find a matching value or the
+inner query ends. This is basically a nested-loop scenario, with the
+only (minimally) redeeming social value being that the planner realizes
+it should pick a fast-start plan for the inner query. I think it should
+be possible to convert this form into a modified kind of join (sort of
+the reverse of an outer join: rather than at least one result per
+lefthand row, at most one result per lefthand row), and then we could
+use join methods that are more efficient than nested-loop. But no one's
+tried to make that happen yet.
+
+ regards, tom lane
+
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+From pgsql-sql-owner+M6000=candle.pha.pa.us=pgman@postgresql.org Mon Dec 17 01:49:56 2001
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+From: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+To: "Tom Lane" <tgl@sss.pgh.pa.us>
+cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
+ "MindTerm" <mindterm@yahoo.com>, <pgsql-sql@postgresql.org>
+Subject: [SQL] 'IN' performance
+Date: Mon, 17 Dec 2001 14:33:40 +0800
+Message-ID: <GNELIHDDFBOCMGBFGEFOEENFCAAA.chriskl@familyhealth.com.au>
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+Status: OR
+
+> In the sub-select form, we pretty much suck: for each tuple in the outer
+> query, we run the inner query until we find a matching value or the
+> inner query ends. This is basically a nested-loop scenario, with the
+> only (minimally) redeeming social value being that the planner realizes
+> it should pick a fast-start plan for the inner query. I think it should
+> be possible to convert this form into a modified kind of join (sort of
+> the reverse of an outer join: rather than at least one result per
+> lefthand row, at most one result per lefthand row), and then we could
+> use join methods that are more efficient than nested-loop. But no one's
+> tried to make that happen yet.
+
+That's what I was thinking...where abouts does all that activity happen?
+
+I assume the planner knows that it doesn't have to reevaluate the subquery
+if it's not correlated?
+
+Chris
+
+
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+From pgsql-sql-owner+M6001=candle.pha.pa.us=pgman@postgresql.org Mon Dec 17 02:00:10 2001
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+ Mon, 17 Dec 2001 01:44:03 -0500 (EST)
+To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+cc: "Stephan Szabo" <sszabo@megazone23.bigpanda.com>,
+ "MindTerm" <mindterm@yahoo.com>, pgsql-sql@postgresql.org
+Subject: Re: [SQL] 'IN' performance
+In-Reply-To: <GNELIHDDFBOCMGBFGEFOEENFCAAA.chriskl@familyhealth.com.au>
+References: <GNELIHDDFBOCMGBFGEFOEENFCAAA.chriskl@familyhealth.com.au>
+Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+ message dated "Mon, 17 Dec 2001 14:33:40 +0800"
+Date: Mon, 17 Dec 2001 01:44:03 -0500
+Message-ID: <29730.1008571443@sss.pgh.pa.us>
+From: Tom Lane <tgl@sss.pgh.pa.us>
+Precedence: bulk
+Sender: pgsql-sql-owner@postgresql.org
+Status: OR
+
+"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
+> That's what I was thinking...where abouts does all that activity happen?
+
+The infrastructure for different join rules already exists. There'd
+need to be a new JOIN_xxx type added to the various join nodes in the
+executor, but AFAICS that's just a minor extension. The part that is
+perhaps not trivial is in the planner. All the existing inner and outer
+join types start out expressed as joins in the original query. To make
+IN into a join, the planner would have to hoist up a clause from WHERE
+into the join-tree structure. I think it can be done, but I have not
+thought hard about where and how, nor about what semantic restrictions
+might need to be checked.
+
+ regards, tom lane
+
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projects / postgresql / commitdiff